Process for preparing vinyl acetate

ABSTRACT

PROCESS FOR PRODUCING VINYL ACACETATE WHICH COMPRISES SUBJECTING A FEEDSTOCK CONSISTING ESSENTIALLY OF ETHYLENE GLYCOL DIACETATE TO PYROLYSIS IN THE VAPOR PHASE AT A MASS VELOCITY IN EXCESS OF 200 LBS./HR./FT.2 OF PYROLYSIS ZONE CROSS-SECTIONAL AREA.

April 16, 1974 RESIDENCE TIME, SECONDS Filed June 22, 1973 PYROLY SISZONE TEMPERATURE, DEGREES KELVIN United States Patent US. Cl. 260-491 14Claims ABSTRACT OF THE DISCLOSURE Process for producing vinyl acetatewhich comprises subjecting a feedstock consisting essentially ofethylene glycol diacetate to pyrolysis in the vapor phase at a massvelocity in excess of 200 1bs./hr./ft. of pyrolysis zone cross-sectionalarea.

CROSS'REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of co-pending application Ser. No. 83,221, filedOct. 22, 1970, now abandoned.

BACKGROUND OF THE INVENTION Vinyl acetate is a large-scale article ofcommerce having wide applicability in the production of polymericcompounds. Traditionally, vinyl acetate has been prepared by thereaction between acetylene and acetic acid.

More recently, the reaction of ethylene, oxygen and acetic acid has beenused. Both these processes, however, suffer from disadvantages.Acetylene is an expensive raw material and also is hazardous to handle.The ethylene-based processes involve oxidations which are relativelydifiicult to operate safely; and, in some instances, have suffered fromserious corosion problems.

Among other alternate routes to vinyl acetate considered in the priorart, one offering substantial promise employs ethylene glycol diacetateas the feed. However, prior art attempts to pyrolyze (i.e., thermallycrack) this material to vinyl acetate have not found acceptance becauseof low selectivities. Thus, for example, while Chitwood, U.S. Pat. No.2,251,983 proposed to produce vinyl acetate by such a pyrolysis in 1941,the selectivities obtained were under 70%, a value too low to make theprocess economically viable. More recent work (see Shibahara, Yoshijimaand Tsutsumi, Technol. Reports, Osaka University, 11, No. 482 (1961), p.405) failed to demonstrate significantly increased selectivity; indeed,the results reported in this relatively recent work were in largemeasure inferior to Chitwoods earlier work and in only one isolatedexperiment was a selectivity of even 80% obtained. The prior art,therefore, would seem to indicate that an economically viable processfor the production of vinyl acetate by pyrolysis of ethylene glycoldiacetate would not be practicable because yields resulting from suchpyrolysis were relatively low.

SUMMARY OF THE INVENTION It has, however, now been found thatselectivities in ethylene glycol diacetate pyrolysis to vinyl acetate of80-90% and even higher are readily obtained when the feedstock issubjected to vapor phase pyrolysis in a pyrolysis zone at a feedstockmass velocity in excess of 200 lbs./hr.ft. of pyrolysis zonecross-sectional area. It has also been found that best yields across thepyrolysis zone are obtained when the time-temperature characteristics ofthe pyrolysis are carefully controlled, although good yields can beobtained even when the pyrolysis occurs under other time-temperatureconditions. In practice, the feedstock is forced through the pyrolysiszone by means of a pressure gradient, i.e., a pressure differentialbetween pyrolysis zone inlet pressure and pyrolysis zone outletpressure. In preferred practice, external heat (i.e., heat from anexternal source) is supplied to the materials being pyrolyzed during thepyrolysis. This requires that the surface of the pyrolysis zone incontact with the material being forced through the pyrolysis zone(feedstock, reaction products and reaction diluents, if any) bemaintained at a higher temperature than the bulk temperature of thematerial flowing through the pyrolysis zone to provide the thermalgradient necessary for the heat input. Preferably, though notessentially, the temperature of the surface in contact with the materialflowing through the pyrolysis zone (the skin temperature) is controlledso as not to exceed the bulk temperature by more than 25 C.

DETAILED DESCRIPTION OF THE INVENTION The process of this inventioninvolves the vapor phase pyrolysis of a feedstock consisting essentiallyof ethylene glycol diacetate to vinyl acetate. In the pyrolysis theethylene glycol diacetate is converted to a mixture containing, inaddition to unconverted ethylene glycol diacetate, vinyl acetate, aceticacid and, of course, some by-products.

The. source of the ethylene glycol diacetate is not of substantialimportance to this invention, and it can be prepared, for example, byroutine esterification of ethylene glycol with acetic acid. However, apreferred manner for preparing this material is by the reaction ofethylene, oxygen and acetic acid in the presence of a halogen or halogencompound in conjunction with a' variable valence metal cation such astellurium, manganese, copper, cobalt and chromium, which metals can besupplied as such or in the form of their salts. The preferred processfor preparation of ethylene glycol diacetate is more fully described inBelgian Pat. No. 733,104, which issued on Mar. 2, 1970.

Preparation of the ethylene glycol diacetate feedstock in this manner,however, provides a material for pyrolysis which often can contain, evenafter purification, some quantity, typically 20% (mole basis) or less ofethylene glycol monoacetate, ethylene glycol, diethylene glycol, anddiethylene glycol diacetate and monoacetate, as well as smaller amounts(up to 1,000-2,000 p.p.m. (weight basis)) of halogenated impurities suchas halohydrin, ethylene dihalide, ethylene haloacetate, diethyleneglycol monohalide and diethylene glycol haloacetate. All of theforegoing impurities are permissible components in the ethylene glycoldiacetate feed to pyrolysis, and it is for this reason that thefeedstock to the pyrolysis is described as one consisting essentially ofethylene glycol diacetate. It should be noted, however, that theseimpurities, during pyrolysis, can yield some vinyl acetate but theirselectivities to vinyl acetate in such pyrolysis are very much lowerthan that observed with ethylene glycol diacetate itself.

Additionally, it is particularly preferred to employ feedstockcontaining the minimum practicable amount of halogenated materials sincethe halogenated materials introduced not only create corrosion problemsbut also appear to somewhat reduce the overall selectivity to vinylacetate in the pyrolysis. Accordingly, while it is permissible to employfeedstock to pyrolysis containing 1,000- 2,000 p.p.m. of halogenatedimpurities, it is generally more desirable to employ feeds containingunder 200 p.pm. of such impurities, and it is preferred to employfeedstocks containing 10-20 p.p.m. of such halogenated impurities oreven less.

Moreover, in continuous commercial operation, the pyrolysis wouldnormally be carried out on a partial conversion basis, i.e., only fromabout 5% to about 60% of the ethylene glycol diacetate will be convertedper pass through the pyrolysis zone and unconverted material, togetherwith by-products unavoidably formed during previous passes of thefeedstock through the pyrolysis zone, would be recycled. Such byproductscan thereby build up in the feedstock to an appreciable extent and, eventhough purged in part, can accumulate to a point amounting to as much as(mole basis) of the total feed. Such by-products that can build up inthis manner include additional ethylene glycol monoacetate, acetone,acetaldehyde, acetic anhydride, ketene, ethylidene diacetate andhigh-boiling materials of unknown structural formulae.

Finally, it should also be recognized that ethylene glycol diacetate isa high-boiling liquid which requires high temperature or low pressure orboth to be readily volatilized. To assist in such volatilization, it isoften advantageous to introduce low-boiling materials duringvaporization and prior to pyrolysis. Any material which is relativelyinert under pyrolysis zone conditions can be used for this purpose.These materials include gases such as nitrogen, argon, helium and carbondioxide, as well as light parafiins such as methane, ethane, propane andbutanes. Also suitable and preferred as a diluent is acetic acid sinceit does not introduce any extraneous material to the system. Acetic acidis substantially inert under pyrolysis zone conditions except that itmay undergo partial dehydration to acetic anhydride, such dehydration isnot in any way deleterious to the overall process. Whichever inert(including, in this context, acetic acid) is employed, it can beemployed in amounts as little as 0.5-1.0 mole percent and up to amountsas great as 70-80 mole percent. It is normally desired to employ amountsof inert from about 1% to about 60% to facilitate ethylene glycoldiacetate volatilization, and it is preferred to employ inerts in anamount between about 3% and about 50%, all these percentages being on amole basis.

Despite the presence of the wide variety of material in the feedstock tothe pyrolysis as outlined on the preceding paragraphs, the feedstock isnevertheless characterized as one consisting essentially of ethyleneglycol diacetate since it is this material which undergoes highlyselective pyrolysis to vinyl acetate and, except as hereinabove noted,these other materials display no adverse effect upon the pyrolysis and,in some instances, are indeed favorable to the pyrolysis. In normalpractice one would use feedstocks containing (exclusive of lightdiluents added to facilitate volatilization) 50100% (mole basis),desirably 75l00% (mole basis) and preferably 80-100% (mole basis) ofethylene glycol diacetate.

As hereinabove indicated, the critical feature associated with obtaininghigh selectivities in the conversion of ethylene glycol diacetate tovinyl acetate by pyrolysis is the mass velocity (computed on the basisof contained ethylene glycol diacetate) within the pyrolysis zone. Withthe mass velocity in excess of 200 lbs./hr./ft. of pyrolysis zonecross-sectional area, selectivities in the pyrolysis are substantiallyimproved over those obtainable at lower mass velocities. Further, it hasbeen observed that, within broad limits, the higher the mass velocity,generally the better will be the selectivity up to a mass velocity ofabout 1,000 lbs./hr./ft. of pyrolysis zone cross-sectional area. Beyondthis point further improvements in selectivity are seemingly minor.Accordingly, while any mass velocity in excess of about 200 lbs./hr./ft.of pyrolysis zone crosssectional area is operative and within the scopeof this invention, it is normally desired to operate with massvelocities in excess of 250 lbs./hr./ft. of pyrolysis zonecross-sectional area, and it is preferred to operate with massvelocities in excess of 300 lbs./hr./ft. of pyrolysis zonecross-sectional area.

The upper limit of mass velocity employable within the pyrolysis zone,on the other hand, is not associated with factors directly related toselectivity. As those skilled in the art will realize, the higher themass velocity within the pyrolysis zone, the higher will be the pressuredrop across the pyrolysis zone, i.e., the greater will be the pressuregradient referred to above. As a consequence, the higher the pressuredrop, the higher the inlet pressure will be and the higher the pressurethat the equipment within the pyrolysis zone must be designed towithstand. The consequences of higher design pressures on equipment costand operating cost is manifest; thus, the primary factor affecting theupper limit of mass velocity is an economic one. A somewhat lessimportant but related factor associated with extremely high massvelocities is that the higher the mass velocity, the higher will be thetemperature needed or the greater will be the amount of inert diluentneeded to vaporize the ethylene glycol diacetate-containing feedstock(the alternative of providing compression facilities is feasible butcostly). Accordingly, economic conditions would normally dictate the useof mass velocities through the pyrolysis zone which are less than about500,000 lbs./hr./ft. of pyrolysis zone cross-sectional area, desirablyless than 300,000 lbs./hr./ft. of pyrolysis zone cross-sectional areaand preferably less than 200,000 lbs./hr./ft. of pyrolysis zonecross-sectional area.

Other conditions within the pyrolysis zone are conventional. Thus, forexample, pyrolysis temperatures between about 435 C. and about 560 C.and preferably between about 445 C. and about 550 C. are employed.Residence times within the pyrolysis zone between about 0.10 and about200 seconds, preferably between about one second and seconds areemployed. Residence times within the pyrolysis zone, as used throughoutthis specification and claims, are determined at the arithmetic averageof inlet and outlet pyrolysis zone temperature and pressure and aredetermined on the basis of feedstock without consideration of increasein the number of moles of gas flowing (and hence in gas velocity) as thefeedstock is pyrolyzed. It should also be noted that the feedstock tothe pyrolysis zone would normally be subjected to one or more preheatingstages prior to pyrolysis, for example, by indirect heat exchange firstwith steam or hot oil and/or by indirect heat exchange with pyrolysiszone effluent. The residence times of the feedstock within such preheatequipment is not part of the residence time within the pyrolysis zone asthis phrase is herein used.

Although the vapor phase pyrolysis reaction is accompanied by anincrease in volume (i.e., one mole of ethylene glycol diacetate, onpyrolysis, theoretically yields one mole of vinyl acetate plus one moleof acetic acid) pyrolysis zone pressure is not a major factor affectingselectiivty. Thus, suitable pyrolysis pressures can be sub-atmospheric,atmospheric, or super-atmospheric, and pressures of from about 0.1p.s.i.a. up to p.s.i.a. or even higher can readily be employed. Whenpyrolysis zone pressures higher than about 115 p.s.i.a. are employed, itis normally preferred to dilute the ethylene glycol diacetate-containingfeedstock with a diluent (preferably acetic acid) to reduce the partialpressure of ethylene glycol diacetate within the pyrolysis zone to avalue less than 115 p.s.i.a. Additionally, operation of the pyrolysiszone with subatmospheric outlet pressures is less preferable thanoperation with higher outlet pressures because equipment designed forsub-atmospheric pressure operation tends to be more expensive.Accordingly, it is generally desired to operate with pyrolysis zonepressures between about atmospheric and about 115 p.s.i.a., preferablybetween about 0.5 p.s.i.g. and about 80 p.s.i.g. Additionally, tomaximize ease of equipment design, it is preferred to so configure thepyrolysis zone such that the pressure drop thereacross (i.e., thepressure gradient or fluid driving force) is between 0.5 p.s.i. andabout 65 p.s.i., preferably between about 0.5 p.s.i. and about 25 p.s.i.

Also as hereinabove indicated, it has been found that highestselectivities in the pyrolysis of ethylene glycol diacetate to vinylacetate are obtained when the timetemperature characteristics of thepyrolysis zone are careand preferably is not in excess of a value suchthat In an especially preferred mode of operation, the residence time isnot less than a value such that and preferably is not in excess of avalue such that In all of the foregoing equations, In is the symbol forthe Napierian or natural logarithm, 0 is the residence time of thefeedstock within the pyrolysis zone in seconds and T represents thearithmetic average of inlet and outlet pyrolysis zone temperaturesexpressed in degrees Kelvin. In the preferred time-temperature regime ofoperation, defined by Equation 1 and 2 above, the value for 0 is 200seconds or less, while T is at least 708 K. (435 C.) but is not greaterthan 833 K. (560 C.). In the especially preferred time-temperatureregime of operation defined by Equation 3 and 4 above, 0 is restrictedto a value of 100 seconds or less, while T is at least 718 K. (445 C.)but not above 823 K. (550 C.).

The annexed drawing, consisting of a single figure, graphicallyreproduces the time-temperature relationships set forth in the form ofequations in the preceding paragraph. In this graphic representation,the ordinate, plotted on a logarithmic scale, is the residence time (0)expressed in seconds, while the abscissa is the arithmetic average ofpyrolysis zone inlet and outlet temperatures (expressed in degreesKelvin) and thus corresponds to T in the foregoing equations. The largerarea in the annexed drawing, denoted by reference letters B-C-D-E-F,represents the preferred regime of controlled time-temperaturecharacteristics defined by Equations 1 and 2 above. The smaller area inthe annexed drawing, denoted by reference letters GH-J-K-L, representsthe especially preferred mode of operations defined by Equations 3 and 4above.

Control of the time-temperature characteristics within the limitsprescribed by the foregoing equations is readily accomplished. Forexample, given an existing pyrolysis zone operating at giventemperatures and residence times, one can compare these temperatures andresidence times with those mandated by the foregoing equations. If thesetemperatures and/or residence times are not Within the ranges indicatedby the foregoing equations, either temperature or residence time or bothare modified to bring these characteristics within the ranges defined bythe foregoing equations to thereby optimize selectivity. In the designand construction of new pyrolysis zone equipment, the configuration ofthe zone is selected so that the temperatures and residence times arewithin the preferred or especially preferred ranges.

The pyrolysis can be conducted catalytically or noncatalytically.Suitable catalysts include activated alumina, vanadia, molybdena,magnesia, alkaline earth pyrophosphates, zinc oxide, cobalt molybdateand the like. These catalysts can be used as such or supported onsuitable inert carriers such as alumina, magnesia, silicon carbide, etc.As in other processes, use of catalysts facilitates 0btention of fasterrates of reaction at lower temperatures.

But in 'view of the increased process complexity introduced 6 by use ofcatalysts and the high selectivities obtained in the absence ofcatalysts, the use of catalysts is not normally preferred.

The configuration of the pyrolysis zone itself is not of substantialimportance to this invention and any convenient type can be employed.Thus, for example, one or a plurality of pyrolysis zones connected inseries or in parallel or both can be employed. The pyrolysis zone itselfcan be in the form of a furnace with the feedstock flowing through thetubes thereof or it can be in the form of a shell-and-tube heatexchanger with the feedstock flowing through either the shell or thetube side thereof. The heating medium employed in the pyrolysis zone isequally of no significance to this invention. Hot gas (e.g., combustionproducts) can be employed as can molten salt or other suitable hightemperature media. Whatever form of pyrolysis zone is adopted, however,it is advantageous to minimize local overheating therein. Since thepyrolysis reaction itself is endothermic, although only slightly so(7000-7500 cal. per gram mole of ethylene glycol diacetate pyrolyzed)and thus requires heat input, the point at which such local overheatingis most likely to occur is at the wall of the pyrolysis zone in contactwith the feedstock, especially at points nearer to the outlet thereof.It is therefore desirable though not essential, to so configure thepyrolysis zone that the skin temperature is not more than 25 C. higherthan the bulk temperature of the feedstock flowing through the pyrolysiszone and preferably not more than about 15 C. higher than this bulktemperature.

As used throughout this specification and in the claims, the phraseologybulk temperature is intended as synonymus with the so-called mixing-cuptemperature; see Jakob, Heat Transfer, vol. I, p. 422 et seq., J. Wiley,New York (1959). Throughout this specification and in the claims,temperatures referred to are bulk temperatures unless otherwise stated.

It will readily be appreciated that the process of this invention isespecially suitable for large-scale continuous commercial operationalthough batch or semi-batch operations are also possible.

Once the pyrolysis is completed, the pyrolysis zone effluent is cooledand vinyl acetate is separated therefrom in conventional manner as, forexample, by fractionation. As hereinabove indicated, unconvertedfeedstock can be recycled to the pyrolysis zone. The manner in which thepyrolysis zone efiluent is cooled does not appear to be a criticalfactor to optimal vinyl acetate yields. Thus, cooling may be by rapidquenching with water or other relatively stable media or by suchinherently slow techniques as heat exchange between pyrolysis zoneefilruent and pyrolysis zone feed.

EXAMPLES Moles ethylene glycol diacetate reacting Moles ethylene glycoldiacetate fed Selectivity-the ratio (expressed as a percent) of:

Moles vinyl acetate formed Moles ethylene glycol diacetate reactingYield-the ratio (expressed as a percent) of:

Moles vinyl acetate formed Moles ethylene glycol diacetate fed Example IEthylene glycol diacetate is prepared by refluxing ethylene glycol andexcess acetic acid in the presence of 1% (wt. basis) p-toluene sulfonicacid as a catalyst at atmospheric pressure until water liberation ceases(approximately 4 hours). During this operation, make-up acetic acid isperiodically added to replace that lost with the water removed. Theresultant material is then vacuum distilled to remove unreacted aceticacid, ethylene glycol and by-product ethylene glycol monoacetate andthen redistilled under vacuum to produce a dry ethylene glycol diacetatefeed of 99% purity, the principal impurity being ethylene glycolmonoacetate.

This feedstock is then charged as a liquid to an electrically heated 1inch diameter steel tube which acts as a vaporizer. To facilitate heattransfer, this tube is packed with glass beads. The now-vaporizedethylene glycol diacetate is then passed to one of two differentlyconfigured pyrolysis zones denominated in the following table as reactorA and B. Reactor A is a 1 inch outer diameter, 13 BWG tube, 4 ft. inlength to which heat is supplied by circulating molten salt through ajacket surrounding the tube. Reactor B is an unpacked M4 inch outerdiameter, 20 BWG tube 20 ft. in. length shaped into a spiral and totallyimemrsed in a constant temperature molten salt bath. The series of runsdescribed in the following table are carried out at a varying massvelocity of ethylene glycol diacetate within the pyrolysis zone but witha constant pyrolysis zone inlet temperature of 500 C. and pressure of 50p.s.i.g. (temperature maintained as necessary by bypassing cold ethyleneglycol diacetate around the vaporized). Each run is continued for atleast one-half hour or longer if necessary to attain steady-stateoperation. Pyrolysis zone eflluents are analyzed by gas chromatography.

TABLE I Percent Run. Average Time, Con- Selecvelocity, N0. Reactortemp., 0. sec. version tivity lbs./hr.it.'

1 Results are those reported by Shibahara et al., Technol. Reports,Osaka University, 11, No. 482 (1961) page 405.

9 Results are those reported by Chitwood in U.S. Patent No. 2,251,983.

Mess velocity refers to lbs. per hour of contained ethylene glycol persq. foot of pyrolysis zone cross-sectional area.

The lettered runs of the above table are controls, not illustrative ofthe invention. The results of the numbered runs tabulated above, whichare illustrative of the invention, indicate that as mass velocityincreases, selectivity also increases (albeit slightly) until, at abouta mass velocity of ca. 1,000 (contrast runs 1, 4 and 9) it tends tolevel 01f and remain at a high level as mass velocity is furtherincreased.

When runs similar to those described above are carried out at highermass velocities, comparable results are obtained. For example, a run ata mass velocity of 20,000 lbs./hr./ft. at 511 C. for a residence time of10 seconds displays a conversion of 35% and a selectivity of 85%. Asimilar run at a mass velocity of 80,000 lbs./hr./ft.'* at 520 C. for2.3 seconds displays a 12% conversion and a selectivity of 92%. Otherruns at 546 C. with mass velocities of 200,000 (time of one second) and500,000

l bs./hr./ft. (time of 0.4 second) give conversions of 15% and 10%respectively and respective sele'ctivities of 92% and 95% Example H Runs4, 7 and 10 of Example I are repeated employing ethylene glycoldiacetate prepared by reacting ethylene dichloride and sodium acetateaccording to the procedure of Shibahara et al., Technol. Repts., OsakaUniversity, 11, No. 482, p. 405 (1961) and also by the reaction at C.for 3 hours of glacial acetic acid and ethylene in the presence ofpalladous nitrate, sodium nitrate and cupric acetate (see Example 3 ofBritish Pat. No. 1,124,862). In both instances the distillationprocedures of Example I are followed to produce feeds of comparableethylene glycol diacetate purity. Pyrolysis results are substantiallyidentical. This indicates that the process of this invention isapplicable to ethylene glycol diacetate feeds of any source.

These runs are also repeated with ethylene glycol diacetate prepared asin Example I to which is added varying amounts of halogenated compounds(an equimolar mixture of bromohydrin, ethylene dibromide, ethylidenedibromide and ethylene glycol bromoacetate or an equimolar mixture ofthe chlorine analogues of these materials). When 1% (wt. basis) ofeither mixture is added, a significant decrease in selectivity is noted.When only 20 p.p.m. are added, the selectivity decrease is of the orderof 45% which is often acceptable. With 2 p.p.m. added, the selectivitydecrease appears to be under 1% while no significant selectivitydecrease is noted when lesser amounts are added.

Example III A series of runs are carried out in a 1 inch outer diameter(15 BWG) steel tube of 80 feet length. The tube is shaped into a coiland completely submerged in a constant temperature salt bath. Feedmaterial is preheated and vaporized prior to being fed to the reactor ina vaporizer similar to that described in Example I. Reactor off-gas isanalyzed by gas chromatography. Those runs identified by number areillustrative of this invention since the timetemperature characteristicsare within the parameters set forth above while lettered runs falloutside of these ranges and illustrate the somewhat poorer resultsobtained.

TABLE III Res. Mess Percent time, velocity, sec. lbs./hr./it. ConversionSelectivity It will be noted that selectivity obtained in control C isquite high but the conversion in this control is so low as to rendersuch a process normally uneconomic except in special situations.

'Example IV Runs 4, 7 and 10 of Example I are again repeated but atinlet pressures of 20 p.s.i.g., 50 p.s.i.g. and 200 p.s.i.g. In the runsat 200 p.s.i.g., vaporization of the ethylene is facilitated by admixing10%, 20% and 50% of acetic acid with the ethylene glycol diacetate. Theruns at 200 p.s.i.g. are also repeated employing sufiicient nitrogenbubbled through. the ethylene glycol diacetate feed to produce vaporadmixtures of the same overall compositions. Results in all these runsare substantially similar to the corresponding runs of Example I.

Example V A feedstock consisting essentially of ethylene glycoldiacetate is prepared according to the teachings of Belgian Pat. No.738,104 by continuously reacting ethylene, oxygen and acetic acid in thepresence of tellurium added to the system in the form of telluriumdioxide and a bromide added to the system as HBr and fractionating thereaction eifiuent to produce a material of the following approximatecomposition:

Ethylene glycol monoacetate 1%.

Ethylene glycol diacetate 99%.

Ethylene glycol Trace.

Diethylene glycol p.p.m. (wt. basis).

Small amounts (ca. p.p.m.) of brominated impurities are also present andinclude bromohydrin, 2- bromoethanol and diethylene glycol diandmono-bromides. This stream is continuously fed to a vaporizer at a rateof 5.90 parts per hour. Also fed to the vaporizer are 26.0 parts perhour of a recycle stream obtained as subsequently described which hasthe following composition:

1 180 p.p.m. (wt. basis).

The admixture formed by mixing the feedstock and recycle streams isvaporized at 60 p.s.i.a. and 225 0., preheated by indirect heat exchangewith pyrolysis zone efliuent to a temperature of 465 C. and then fed tothe pyrolysis zone. The pyrolysis zone is in the form of a furnacethrough which the admixture flows at a mass velocity of 106,500lbs./hr./ft. of pyrolysis zone crosssectional area (i.e., furnace tubecross-sectional area). During pyrolysis, the admixture is heated to 535C. and the residence time within the pyrolysis zone is 2.1 seconds. Thepyrolysis zone is arranged so that inner tube wall temperature is notmore than 10 C. higher than bulk stream temperature toward the hot(outlet) end of the pyrolysis zone. The effluent from the pyrolysis zone(37.89 parts/hr.) has the following approximate composition:

The pyrolysis zone effluent also includes small quantities of higherglycols (diand triethylene glycol) as well as small amounts ofbrominated hydrocarbons (e.g., bromohydrin, ethylene dibromide andethylidene dibromide) as well as other materials such as ethylidenediacetate. This eflluent is cooled to 316 C. by heat exchange With thepyrolysis zone feed and, now at a pressure of 40 p.s.i.a., is furthercooled to 160 C. prior to treatment for product recovery.

Product recovery is accomplished by a sequence of distillations whereinlow-boiling materials (including unidentified low-boiling materials andcarbon oxides) are first removed. Then, in a subsequent distillation,materials boiling at a higher temperature than vinyl acetate are removedas a bottoms product and recycled by pyrolysis zone feed. This bottomsis the source of the recycle stream admixed with the feedstock topyrolysis. The overhead, containing the vinyl acetate, is distilled toremove light materials, predominately acetone and heavy materials suchas acetic acid. Vinyl acetate product of greater than 99% purity (5.08parts/hr.) is obtained. Acetic acid by-product can, of course, be usedto prepare additional ethylene glycol diacetate containing feedstock.Acetone, the principal product of side reactions, may also be purifiedand sold.

From the foregoing, it will be noted that ethylene glycol diacetateconversion across the pyrolysis zone itself is approximately 19.5% pcrpass, selectivity across the pyrolysis zone is over 89.5% per pass,which also represents overall yield, neglecting losses in recovery.

In a similar operation the foregoing example is repeated except thatwall temperature is increased by adjusting heating rate so that walltemperature in the pyrolysis zone is as much as 40 C. higher than bulkstreamtemperature. Conversion increases slightly to about 21% per passbut selectivity falls to about 86%. Additionally later examination ofthe pyrolysis zone indicates deposition of carbonaceous materials on thewall thereof.

What is claimed is:

1. in a process for the vapor phase pyrolysis of a feedstock consistingessentially of ethylene glycol diacetate to vinyl acetate, theimprovement which comprises pyrolyzthe feedstock within a pyrolysis zonewherein the pyrolysis is conducted at a temperature between about 435 C.and about 560 C. for a residence time of the feedstock within thepyrolysis zone from about 0.1 second to about 200 seconds, the feedstockmass velocity being at least 200 lbs./hr./ft. of pyrolysis zonecross-sectional area.

2. In a process for the vapor phase pyrolysis of a feedstock consistingessentially of ethylene glycol diacetate to vinyl acetate, theimprovement which comprises forcing the feedstodk through a pyrolysiszone at a mass velocity of at least 200 lbs./hr./ft. of pyrolysis zonecrosssectional area by means of a pressure differential betweenpyrolysis zone inlet and outlet pressures while supplying external heatto the surface of the pyrolysis zone in contact with the material beingforced through the pyrolysis zone and conducting the pyrolysis at atemperature between about 435 C. and about 560 C. for a residence timebetween about 0.1 second and about 200 seconds, the difference betweenthe temperature of the surface of the pyrolysis zone in contact with thematerial being forced through the pyrolysis zone and the bulk fluidtemperature within the pyrolysis zone not exceeding 25 C. 3. A processin accordance with claim 2 wherein the temperature difference is notgreater than 15 C.

4. A process in accordance with claim 2 wherein the pressuredifferential between pyrolysis zone inlet and outlet pressures isbetween about 0.5 p.s.i. and about 65 p.s.i.

5. A process in accordance with claim 2 wherein the pressuredifferential between pyrolysis zone inlet and outlet pressures isbetween about 0.5 p.s.i. and about 65 p.s.i. and the temperaturedifference is not greater than 15 C.

6. A process in accordance with claim 1 wherein the feedstock containsat least 70 mole percent ethylene glycol diacetate and not more than 200p.p.m. (wt. basis) of halogenated impurities.

7. A process in accordance with claim 6 wherein the feedstock containsless than 20 p.p.m. of halogenated impurities.

8. A process in accordance with claim 1 wherein the mass velocity is atleast 250 lbs./hr./ft.

9; A process in accordance with claim 1 wherein the mass velocity is'atleast 300 lbs./hr./ft.

1 1 12 10. A process in accordance with claim 1 whereinthe 14. A processin accordance with claim 13 wherein the mass velocity does not exceed500,000 lbs./hr,/ft. 1,1 residence time is not less than'a 'value suchthat 11. A process in accordance with claim 1'. wherein the v t H massvelocity is between about 250 and about 300,000 1 a}- Q-J31. 700a1bs./hr./ft. 5 T ,j 12. A process in accordance with claim 1 wherein,the 3 5 n 3 9 5 O a val e su h t at mass velocity is between about 300and about 200,000 e 000; a. lbs./hr./ft. a I In e= -.29.0945

13. A process in accordance with claim 1 wherein the residence timewithin the pyrolysis zone is not less than 11 t0- value such that 1 ifnot being in excess of 100 seconds andT being at least 718 K. but notabove 823 K.

References I 27,940 V. T 3 a, I UNITED 'STATESPATENTS i 1 2,251,923?871941 Chitwood' zoo-491 and is not in excess of a value such that we:-10THERREFERENCBS a:

- Q I .Yoshijima et al:: Techno1. Reports, Osaka Univ., 1 1, T No. 4821961, p. 405., a

20 v a wherein In is the symbol for the Napierian. logarithm, VIVIANGARNER, Primary Examiner Us. c1.'x.R.

0 is the residence time of the feedstock within the pyrolysis zone inseconds and T is the arithmetic average of p'yroly; sis zone inlet andoutlet temperatures in degrees Kelvin}. 260497 A, 499

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION- Patent No. 3804 887. v Dated A r- Q 9Z4 Robert Hoch et al.

lnventor(s) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column l, line 65, "lbs./hr'."f,t. should read lbs ./hr. ft. Column 2,line 34, "The preferred" should read This preferred Column 4, line49,"'tiivty" should read tivity Column line 23, "imemrsed" should readimmersed. line 30, "vap0ried)." should read vaporizer}. column 7,

Table I, second figure in "'Average temp., C." column reads "500 shouldread 1- 550 same table I, note 3 refers to "ethylene glycol per sq.foot" should read ethylene glycol diacetateper' sq. foot. Column 9, line61, the percent for high boiling materials i shown as Q.0l" should read0.10. Column 10, line 6, "'recycled by pyrolysis" should read recycledto pyrolysis. line 34, "pyrolyz" should read pyrolyzi ng.

Signe and d this 24th day of September 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR. v r C. MARSHALL DANN Attesting Officer Commissionerof Patents FORM PO-105O (10-69) U'SCOMM DC 6037M,

' I a 11.5. GOVERNMENT PRINTING OFFICE I969 0-366-334.

